1. Field of the Invention
The present invention relates to a baked flux for submerged arc welding and, more specifically, to a baked flux of high basicity for use in welding low-alloyed steels such as high-tensile steels, low-temperature steels and high-temperature steels, capable of depositing low-hydrogen and high-toughness weld metals in combination with various welding wires, and satisfactorily facilitating welding work.
2. Discussion of the Background
The recent increasing trend of the size of weldments has further enhanced the necessity of welding very thick high-tensile steel plates, and the requirement of low-temperature toughness of weld metals has become more severe. Accordingly, in constructing such a large weldment through submerged arc welding, it is necessary to use an optimum welding wire of appropriate composition to deposit a weld metal having sufficient tensile strength and toughness, and to use a flux capable of reducing oxygen content and diffusible hydrogen content of the weld metal and enabling safe welding work.
Fluxes for submerged arc welding are classified by manufacturing method roughly into three types, namely, fused fluxes, sintered fluxes and baked fluxes. The fused flux has been generally used from long ago. The fused flux is manufactured by uniformly fusing a material containing SiO.sub.2, MnO, CaO, MgO and CaF.sub.2 as principal components at a temperature of 1300.degree. C. or above, cooling the material, and then crushing the fused and cooled material in an appropriate particle size according to purpose. The sintered flux is manufactured by solidifying a mixture of powder materials by water glass, granulating the solidified mixture in an appropriate particle size, and then heating the particles at a high temperature in the range of 800.degree. to 1000.degree. C. to remove the water of crystallization and volatile matters from the particles. The baked flux is manufactured by solidifying a mixture of a powder material, an alloying agent, a deoxidizing agent and an arc stabilizer by water glass, granulating the solidified mixture into particles, and then heating the particles at a temperature on the order of 500.degree. C.
The baked flux, in particular, among those fluxes is able to contain carbonates such as CaCO.sub.3 and BaCO.sub.3. The metal carbonates contained in the baked flux are decomposed to produce carbon dioxide gas, which reduces the steam partial pressure of the arc atmosphere and remarkably reduces the diffusible hydrogen content of the weld metal as compared with other fluxes. As mentioned above, since the baked flux contains appropriate amounts of alloying agent and deoxidizing agent, and the alloying agent and the deoxidizing agent are not burnt during the manufacturing process, the baked flux enables the comparatively easy achievement of the adjustment of the chemical composition, deoxidization and fine crystallization of the weld metal.
However, the baked flux has some disadvantages as well as the above-mentioned advantages. As is disclosed in Japanese patent publication Nos. 56-53476 and 56-53477, the baked flux contains a large amount of MgO to increase the basicity. Accordingly, the baked flux is liable to cause weld defects, such as the deterioration of slag removability in the vicinity of the root of grooves, formation of convex beads and slag inclusion. Furthermore, as is disclosed in Japanese patent publication No. 57-31516, strict welding conditions must be satisfied to secure requisite properties of the weld metal, and hence the scope of the application of the baked flux is limited.
In addition to those problems, the conventional baked flux has a disadvantage that the baked flux is unable to provide expected effect unless the baked flux is used in combination with a welding wire having a specific composition. Accordingly, in view of the management of practical welding work, it has been desired to develop a flux which is applicable universally to welding various kinds of steel plates.